Electrochemical cells provide electrical energy that powers a host of electronic devices such as external and implantable medical devices. Among these many medical devices powered by electrochemical cells are external medical drills and implantable cardiac defibrillators. Such medical devices generally require the delivery of a significant amount of current in a relatively short duration of time. Thus, these devices typically require the use of electrochemical cells that comprise an increased delivery capacity and an increased rate of charge delivery. As defined herein, “delivery capacity” is the maximum amount of electrical current that can be drawn from a cell under a specific set of conditions. The terms, “rate of charge delivery” and “rate capability” are defined herein as the maximum continuous or pulsed output current a battery can provide per unit of time. Thus, an increased rate of charge delivery occurs when a cell discharges an increased amount of current per unit of time.
Cathode chemistries such as carbon monofluoride (CFx) have been developed to provide increased discharge capacities that meet the power demands of external and implantable medical devices. CFx cathode material is generally known to have a discharge capacity of about 875 mAh/g, which is well suited for powering implantable medical devices over long periods of time. However, electrochemical cells constructed with cathodes comprised of carbon monofluoride are generally considered to exhibit a relatively “low” rate capability. For example, electrochemical cells constructed with lithium anodes and CFx cathodes typically exhibit rate capabilities from about 0.5 mA/cm2 to about 3 mA/cm2. As such, electrochemical cells constructed with Li/CFx couples are generally well suited for powering electrical devices, like an implantable cardiac pacemaker that require power over long periods of time at a relatively low discharge rate.
In contrast, electrochemical cells constructed with lithium anodes and cathodes comprising silver vanadium oxide (SVO) are generally considered to exhibit a relatively “high” rate capability. Lithium cells constructed with SVO cathodes, in contrast to CFx cathodes, generally exhibit rate capabilities that range from about 25 mA/cm2 to about 35 mA/cm2. As such, lithium electrochemical cells constructed with cathodes comprised of SVO are generally well suited to power devices that require an increased rate capability, such as an implantable cardiac defibrillator. However, lithium cells constructed with cathodes comprising SVO typically have a lower discharge capacity as compared to those having cathodes comprising CFx. Silver vanadium oxide cathode material is generally known to have a discharge capacity of about 315 mAh/g, which is significantly less than the discharge capacity of 875 mAh/g for CFx as previously discussed. Therefore, what is desired is an electrochemical cell having an electrode design that comprises both a relatively “high” discharge capacity material and a relatively “high” rate capability material that is capable of providing increased discharge capacity at a relatively high rate.
Prior art electrochemical cells comprising a lithium anode and a cathode constructed with both CFx and SVO materials are disclosed in U.S. Pat. No. 6,551,747 to Gan, which is assigned to the assignee of the present application and incorporated herein by reference. These cells are well suited for powering implantable medical devices, such as implantable defibrillators, that require a relatively high charge capacity with an increased discharge rate. The present invention provides a lithium electrochemical cell comprising a sandwich electrode design that incorporates both relatively high discharge capacity and relatively high rate capability materials similar to that described by the Gan '747 patent, but having an increased energy density and improved rate capability in comparison to prior art cells.
In addition, the present invention provides for an efficient assembly process that is more conducive for manufacturing. Prior art electrochemical cells, such as those disclosed in the Gan '747 patent, are assembled using a number of time consuming manual process steps. The assembly process of the present invention provides for a more efficient process that eliminates many of the time consuming manual steps of the prior art assembly process, thereby reducing manufacturing time and expense.